Based on earlier models developed for polyaddition of diisocyanates and polyols, a non-stoichiometric process was provided for step-by-step polymerisation of 2,4- and 2,6-tolylene diisocyanate (TDI) with diols which had various molecule sizes and the nature of polyethers and polyesters. Said process yielded urethane oligomers which had -NCO or -OH groups as their chain end groups. After elimination of excess monomer, these compounds were-at subsequent stages (2-5)-subjected to further reaction with diisocyanate or with selected polyol. The process was operated in bulk and excess monomers were eliminated with the use of the selective extraction method. Linear products were obtained in that way which had well defined chain structures and narrow distribution of their molecular weights (MWD) PD = (M) over bar (w)/(M) over bar (n) = 1.1-1.3. On the basis of IR and mass spectrometry (MS) (electrospray ionization (ESI) and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF)) structural analyses, the expected structures of oligomers were confirmed and the actual compositions of polyurethane mixtures formed at every stage of the polymerisation process could be verified against the data obtained from the model, from the balance calculations (based on determinations of free isocyanate groups) and from the findings of the gel-permeation chromatography (GPC) analysis. Applicability of the presented method was demonstrated and the general scheme was suggested for the process discussed. (C) 2003 Elsevier Ltd. All rights reserved.